High pressure electric discharge device containing a fill of mercury, halogen and an alkali metal and barrier refractory oxide layers



Oct.31, 1967 v ORB.N ETAL 3,350,598

A FILL OF HIGH PRESSURE ELECTRIC DISCHARGE DEVICE CONTAINING MERCURY, HALOGEN AND AN ALKALI METAL AND BARRIER REFRACTORY OXIDE LAYERS Filed Dec. 29, 1965 VICTOR L. CORBIN FREDERIC KOURY MARIE LEVESQUE l N VEN TOR S ATO Y United States Patent HIGH PRESSURE ELECTRIC DISCHARGE DEVICE CONTAINING A FILL 0F MERCURY, HALOGEN AND AN ALKALI METAL AND BARRIER RE- FRACTORY OXIDE LAYERS Victor L. Corbin, Salem, Frederic Koury, Lexington, and Marie Levesque, Peabody, Mass., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Dec. 29, 1965, Ser. No. 527,407 9 Claims. (Cl. 313220) ABSTRACT OF THE DISCLOSURE Alkali metal atoms tend to migrate through quartz arc tube walls during operation of the lamp. To prevent such migration, a barrier layer of zirconium oxide is placed upon the quartz wall. To prevent the zirconium oxide atoms from migrating into the arc tube, a barrier layer of a refractory oxide is placed over the zirconium oxide coating.

This invention relates to high pressure electric discharge devices and particularly those which emit light other than that which is generally produced by a device containing mercury alone as the fill. In particular, this invention relates to high pressure electric discharge devices Which contain mercury, a halogen, a light emitting metal and an alkali metal in a quartz arc tube.

In the prior art, high pressure electric discharge devices have been manufactured which contain a fill of mercury alone. When a potential is imposed across the electrodes in the device, the mercury is ionized and emits its characteristics spectral lines, generally in the bluegreen region. It has recently been discovered that these devices could be modified in their emission by the inclusion of metals other than mercury so that the light produced was the combined emission of mercury and the included metals. Blending of emission in this manner can produce wide variation in colors and most importantly, a white light can be attained.

With these devices, an alkali metal, generally sodium has been included to fatten the arc and insure reasonable operating voltage. Unfortunately, the sodium atoms tend to migrate quite readily through the quartz of the arc tube and are unavailable to fulfill their role in the lamps operation. The atomic radii of the sodium atoms is such that the quartz is porous to the atoms. Hence during lamp operation, the sodium will pass from the hot are plasma through the quartz envelope to the outside of the arc tube where it is cooler. When this happens, the arc becomes quite restricted and bows against the quartz and can gradually burn a hole. Even if this extreme condition is not reached due to vast excesses of sodium initially placed in the are tube, the operating voltages of the lamps can increase to a point where the arc cannot be sustained on ordinary ballasts. Moreover, when the sodium migrates, it is unavailable to the discharge and hence will not contribute its characteristic luminous emission spectrum to the device.

According to our invention, we have discovered that sodium migration can be inhibited by coating the inner surface of the arc tube with a material which reduces such migration. Particularly we have discovered that coating a layer of zirconium oxide upon the inner surface of the arc tube will inhibit this migration. However, zirconium oxide alone as a coating on the arc tube has been found to be substantially detrimental to lamp operation and hence, cannot be exposed to the arc stream. We have found that the zirconium oxide layer must be coated with a second layer of refractory to prevent migration of the zirconium atoms into the arc stream. Unless this second layer is included, the zirconium atoms will enter the arc plasma and gradually cause a shift in the radiation to pink, which is the characteristic radiation of zirconium atoms, with a resulting reduction in brightness.

Accordingly, the primary objects of our invention are the reduction of sodium atom migration through the quartz wall of the arc tube of a high pressure electric discharge device and the attainment of reasonable operating voltages and luminous efiiciency during lamp life.

Features of our invention include the coating of a quartz arc tube with a layer of zirconium oxide layer with a second layer of a refractory.

The many other objects, features and advantages of our invention will become manifest to those conversant with the art upon reading the following specification when taken in conjunction with the accompanying drawing wherein the preferred embodiments of our invention are shown and described by way of illustrative examples.

The figure is an elevational view of an arc tube, partial- 1y broken away to expose the inside.

Referring now to the drawing, the arc tube is generally made of quartz. Sealed in the arc tube 12, at the opposite ends thereof, are main discharge electrodes 13 and 14 which are supported on lead-in wires 4 and 5 respectively. Each main electrode comprises a core portion which may be a prolongation of the lead-in wires 4 and 5 and may be prepared of a suitable metal such as for example molybdenum or tungsten. The prolongations of these lead-in wires 4 and 5 can be surrounded by molybdenum or tungsten wire helixes. If desired, a small sliver of a metal or low work function such as thorium, which in this case should be less than about 0.5 mg, can be disposed between the helix and the rod in each of the electrodes to reduce cathode drop. In some cases when its spectrum is not desired, or limitation of the spectrum is preferred, thoriated tungsten wire (containing about 2 t0 4% thorium by weight) can be used or, or course, the thorium can be eliminated entirely.

An auxiliary starting probe or electrode 18, generally prepared of tantalum or tungsten is provided at the base end of the arc tube 12 adjacent the main electrode 14 and comprises an inwardly projecting end of another lead-in wire.

Each of the current lead-in wires described have their ends welded to intermediate foil sections of molybdenum which are hermetically sealed within the pinched sealed portions of the arc tube. The foil sections are very thin, for example approximately 0.0008 inch thick and go into tension without rupturing or scaling off when the heated arc tube cools. Relatively short molybdenum wires 23, 24 and 35 are welded at the outer ends of the foil and serve to convey current to the various electrodes inslde the arc tube 12. The pinched or flattened end portions of the arc tube 12 form a seal which can be of any desired width and can be made by flattening or compressing the ends of the arc tube 12 while they are heated.

The are tube 12 is provided with a filling of mercury which reaches pressures in the order of one half to several atmospheres during lamp operation at temperatures of 450 to 700 C. Moreover the fill contains a halogen, particularly iodine and except fluorine, added as the halogen per se or a halide of one of the metals in the arc tube. As the source of emission other than mercury, we add a light emitting metal as will be described later and as noted above, the alkali metal, particularly sodium is used to control the operating voltage of the arc and its characteristics. The amount of mercury added can be varied widely as we have indicated but we prefer to add approximately 2.5 10 to 4.1 gram atoms of mercury per centimeter of arc length. However, substantial light emission can be obtained at filling pressures less than the above stated limits. Furthermore, the light emission appears to be substantially independent of the amount of mercury metal added and hence the quantity of the latter can be reduced While lamp operation is still attained. As is conventional in the art, a quantity of rare gas such as helium, argon, neon, krypton or xenon at a pressure of about 25 millimeters of mercury is added to facilitate starting.

In manufacturing arc tubes of lamps containing fills other than mercury alone, many metals can be included such as cadmium, ballium, indium, molybdenum, certain rare earth metals, scandium, thallium, thorium and vanadium. Moreover, each of these lamps should contain an alkali metal, preferably sodium for the reasons described above.

Coated upon the arc tube is a transparent layer 1 of a mixture of zirconium oxide and yttrium oxide, the thickness of which is 400 to 1000 A. The quantity of yttrium oxide which may be included with the zirconium is less than by weight, depending upon the operating temperature of the arc stream. Coated over the zirconium oxide base is a transparent layer 2 of refractory oxide, such as calcium, magnesium or aluminum oxide, generally of 1200 to 3000 A. in thickness. They may be composed of one or more layers or coatings of any of the constituents. Through the use of these coatings, the sodium is inhibited in its migration through the quartz by the action of the zirconium oxide layer. The layer of refractory oxides provides a sheath which prevents the zirconium atoms from migrating into the arc stream. Importantly, the zirconium atoms must be completely shielded from the arc plasma by the second coating to prevent their ionization.

In lamps containing only a layer of zirconium oxide, a non-sustaining arc was obtained having an operating voltage between 180 and 220 v. and produced 23,000 lumens, whereas in the control containing no coatings at all an operating voltage of 147 v. was obtained and the lamp produced 30,000 lumens. Hence, it is apparent that with a lamp containing a zirconium layer alone, the results were poorer than those in which no layer at all was applied. When magnesia was coated over the zirconium oxide layer, the lamp operated at 129 volts and produced 29,000 lumens. At 673 hours, the lamp produced 25,000 lumens and operated at 142 volts. With a coating of alumina, the lamp operated at 129 volts and produced 31,000 lumens. At 621 hours it had a 90% maintenance of 28,000 lumens and operated at 142 volts. A 50-50 percentage by weight mixture of alumina and magnesia as a coating over the zirconia, produced a lamp which initially operated at 144 volts and produced 31,500 lumens and at 871 hours had a maintenance of 81% and an operating voltage of 168 v.

It is apparent that modifications and changes may be made within the scope and spirit of the invention but it is our intention however only to be limited by the spirit and scope of the appended claims.

As our invention, we claim:

1. A high pressure electric discharge device comprising: a quartz arc tube; electrodes disposed at either end thereof; a fill including mercury atoms; halogen atoms and alkali metal atoms; a transparent coating on the inner surface of said arc tube comprising zirconium oxide atoms; and a transparent refractory oxide coating over said zirconium oxide coating.

2. The device according to claim 1 wherein the alkali atoms are sodium.

3. The device according to claim 1 wherein the refractory oxide is at least one member selected from the group consisting of calcium oxide, aluminum oxide and magnesium oxide.

4. The device according to claim 1 wherein yttrium oxide is substituted for zirconium oxide in quantities up to 15% by weight.

5. A high pressure electric discharge device comprising: a quartz arc tube; electrodes disposed at either end thereof; a fill including mercury atoms; halogen atoms and alkali metal atoms; transparent means coated upon the inner surface of said are tube to prevent the migration of said alkali metal atoms through said quartz; transparent means coated over said first mentioned means to prevent the migration of atoms from said first mentioned means into the interior of said are tube when said device is operating.

6. The device according to claim 5 wherein said first mentioned means is zirconium oxide.

7. The device according to claim 6 wherein yttrium oxide is substituted for zirconium oxide in quantities of up to 15 by weight.

8. The device according to claim 5 wherein the alkali metal is sodium.

9. The device according to claim 5 wherein said second mentioned means is at least one member selected from the group consisting of calcium oxide, aluminum oxide and magnesium oxide.

References Cited UNITED STATES PATENTS 1,188,587 6/1916 Von Recklinghausen 313221 2,568,459 9/1951 Noel 313-221 3,234,421 2/1966 Reiling 31325 FOREIGN PATENTS 947,311 1/ 1964 Great Britain.

DAVID J. GALVIN, Primary Examiner.

JAMES W. LAWRENCE, Examiner. C, R. CAMPBELL, Assistant Examiner. 

1. A HIGH PRESSURE ELECTRIC DISCHARGE DEVICE COMPRISING: A QUARTZ ARC TUBE; ELECTRODES DISPOSED AT EITHER END THEREOF; A FILL INCLUDING MERCURY ATOMS; HALOGEN ATOMS AND ALKALI METAL ATOMS; A TRANSPARENT COATING ON THE INNER SURFACE OF SAID ARC TUBE COMPRISING ZIRCONIUM OXIDE ATOMS; AND A TRANSPARENT REFRACTORY OXIDE COATING OVER SAID ZIRCONIUM OXIDE COATING. 